Below certain depths, underwater divers using a compressed breathing gas, are limited, not by their equipment but by the changes which take place in their body chemistry while breathing gas (air) under high pressure. Inert gases, such as nitrogen, enter the tissues of the body at higher rates and reach higher concentrations when breathed under pressure, the solubility going up with increased partial pressure. Prolonged and/or deep dives result in such higher concentrations in the body tissues that the diver must, during ascent to the surface, allow his body to desaturate from its excess nitrogen content. This is commonly referred to as decompression. Too rapid a return to the surface would allow the excess nitrogen, which is in a supersaturated state in the body tissues, to pass beyond the "bubble point." Small bubbles of gas would form in the body tissues, causing the "bends" or Casson disease, one of the worst maladies of diving.
The principal method now employed to guide the diver, during ascent, to insure safe release of excess nitrogen accumulated by his body tissues, is the use of dive tables which specify schedules of decompression pauses during ascents. Such tables are available from the U.S. Navy Experiment Diving Unit, Washington, D.C. and may be found in the "U.S. Diving Manual", NAVSHIPS, 250, 385. These tables are empirically derived and allow the diver a 95% safety factor. That is, if the diver precisely follows the properly selected table, he is 95% sure that the excess nitrogen accumulated by his body tissue will be released with no ill effect.
Derivation of these tables assumes the duration of the dive, from the time the diver entered the water to the time ascent begins, is spent at the maximum depth attained during the dive. Each table, therefore, is a decompression ascent schedule which corresponds to a specified maximum depth, to which a diver has descended and a specific dive duration. In use, a diver must carefully pre-plan his dive. He must select a maximum depth, below which he will not descend, and the specific duration of his dive. These parameters, once determined, allow the diver to select the table providing the appropriate decompression ascent schedule to be followed during ascent. However, since there is a finite number of decompression tables provided, there is a corresponding finite number of maximum depth and dive duration combinations a diver may select. As can be seen, such advance planning substantially limits the diver's ability to alter his dive, once the dive has commenced. Should the diver wish to dive deeper than originally planned or remain submerged longer than planned, he must consult the dive tables once again to determine a new decompression ascent schedule. Such a procedure is obviously impractical.
Repetitive dive situations, that is, where a subsequent dive is to be made soon after (within 24 hours) a prior dive, greatly magnify these problems. Selection of a table containing appropriate decompression ascent information must be made based upon the maximum depth attained, duration of the prior dive, and period of time between completion of the prior dive and initiation of the prospective subsequent dive--in addition to maximum depth and dive duration of the planned succeeding dive. Moreover, table selection for a subsequent dive, in repetitive dive situations, is made regardless of the amount of time elapsed between two succeeding dives--so long as they are made the same day. Thus, even if an extremely large number of tables are provided, the diver is still limited in planning repetitive dives.
Moreover, use of decompression ascent schedules fails to allow a diver to vary the risk factor of his ascent, should he so desire. As pointed out above, the risk factor provided by the U.S. Navy tables is 95%. A diver has no way of determining how to vary any particular decompression ascent schedule so that, if he wishes, he may exceed the 95% safety factor in order to shorten the time required to reach the surface.